Transmission electron microscopy has been applied extensively in the study of nano-scaled structures manufactured in the semiconductor industry. In standard TEM, a large diameter electron spot is used on the specimen and images are projected through an electro-magnetic lens system and captured on a fluorescent screen combined with a CCD camera. In STEM, the specimen is scanned by a fine focused beam and the image is built up synchronously on the basis of transmitted electrons scattered over a range of angles and captured in different angular ranges by dedicated detectors.
Another technique that is known for the analysis of semiconductor devices is Scanning Probe Microscopy (SPM). An example of SPM is Atomic Force Microscopy (AFM), wherein a probe tip that is in contact with the surface of the specimen is scanned over an area of the surface. The tip may be a diamond tip of nano-scaled dimensions, allowing the acquisition of electrical, magnetic, and topographical data from the surface. Other SPM variations are known, all of them involving the use of a probe tip that is scanned over a surface, either in contact mode or in non-contact mode.
The use of SPM probes for scratching a surface and thereby thinning the surface is known as the “Scalpel SPM” technique. By performing consecutive SPM acquisitions on the progressively thinned specimen, a tomographic model may be obtained of the structural and electrical characteristics of a specimen.
3D-tomographic structural data of nano-scaled structures may be also obtained through TEM and in particular through the STEM technique. This is done by tilting the specimen about an axis perpendicular to the beam direction, through a range of tilt angles, allowing one to obtain a sequence of 2D-images of the specimen. The set of 2D-images acquired in the tilt series is then processed by back-projection algorithms which take into account the respective tilting angles and reconstruct the 3D volume of the specimen. The acquisition of the tilt series and the processing of the data is however technically complex.
TEM and SPM are performed on specific specimens, designed, and dimensioned for the purpose of the respective technique. TEM specimens are thin as they need to be electron transparent, whereas contact mode-SPM specimens are more robust given the loads applied to the specimen by the tip. A combination of TEM and SPM has been documented only in very particular cases, as illustrated for example by document U.S. Pat. No. 6,864,483, where TEM is used to study the indentation caused by an AFM probe on an AFM specimen.
One problem that is particular to TEM is the fact that the production of TEM specimens of extremely low thickness is difficult. Milling by Focused Ion Beam (FIB) generally does not allow to thin the specimen to less than about 100 nm, unless complex and costly process steps are applied, such as described for example in U.S. Pat. No. 9,177,760 where a specimen is thinned by FIB to less than 20 nm. The process however involves multiple complex manipulations of the specimen.